x86: Change 4-level page table base address to low memory

At present the 4-level page table base address for 64-bit U-Boot
proper is assigned an address that conflicts with CONFIG_LOADADDR.
Change it to an address within the low memory range instead.

Fixes crashes seen when 'dhcp' on QEMU x86_64 with
"-net nic -net user,tftp=.,bootfile=u-boot".

Reported-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>